Same same gun but each was imported in different configurations.
The ATIs that I have seen were imported as "sporting rifles". No flash hider, high capacity magazine blocks, and a fixed stock welded to a steel trigger housing. There may be other variations out there that I haven't seen, but I think this the most common. I believe there is a thread in the MKE sub-forum posted by a member who just bought one in 5.56. He posted up some pics.
Link..
Found this at a local shop here in our small town. It was “new in box” and “on sale”. I’m new to HK clones or HK in general. After speaking with members on here I was advised to purchase it at that price. The shop owner stated that he has had it since before 2016. All he could tell me about...
www.hkproshop.com
Zeniths were largely imported as pistols with the thought that end users would Form 1 them into SBRs and make them 922r compliant. Personally I feel that the Zenith imports are more easily adapted to SBRs than ATI guns. Everything is already in place and there are no welds or other modifications that have to be dealt with.
IMO your biggest problem is going to be finding one at a reasonable price.